EPR Study of Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Nd<Superscript>143</Superscript> Isotopically Pure Impurity Crystals

The Sc₂SiO₅ single crystals doped with 0.001 at.% of the ¹⁴³Nd³⁺ ion were studied by continuous-wave and pulse electron paramagnetic resonance methods. The g-tensors and hyperfine structure tensors for two magnetically non-equivalent Nd ions were obtained. The spin–spin and spin–lattice relaxation times were measured at 9.82 GHz in the temperature range from 4 to 10 K. It was established that three relaxation processes contribute to the spin–lattice relaxation processes. There are one-phonon spin–phonon interaction, two-phonon Raman interaction and two-phonon Orbach–Aminov relaxation processes. It was established that spin–spin relaxation time is of the same magnitude for neodymium ion doped in Sc₂SiO₅ and in Y₂SiO₅.     

Anomalous magnetism and 209Bi nuclear spin relaxation in Bi4Ge3O12 crystals

The quadrupole ²⁰⁹Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi₄Ge₃O₁₂ single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T ₁ in pure and doped samples. Unlike T ₁, the spin–spin relaxation time T ₂ for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi₄Ge₃O₁₂ with paramagnetic atoms strongly elongated T ₂. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (～30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T ₁ in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi₄Ge₃O₁₂ crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.     

<Superscript>23</Superscript>Na and <Superscript>27</Superscript>Al NMR Study of Structure and Dynamics in Mordenite

Temperature dependencies of ²⁷Al and ²³Na nuclear magnetic resonance spectra and spin–lattice relaxations in mordenite have been studied in static and magic angle spinning regimes. Our data show that the spin–lattice relaxations of the ²³Na and ²⁷Al nuclei are mainly governed by interaction of nuclear quadrupole moments with electric field gradients of the crystal, modulated by translational motion of water molecules in the mordenite channels. At temperatures below 200 K, the dipolar interaction of nuclear spins with paramagnetic impurities becomes an important relaxation mechanism of the ²³Na and ²⁷Al nuclei.     

<Superscript>27</Superscript>Al NMR Study of the Structure and Dynamics of Natrolite

The temperature dependences of nuclear magnetic resonance and magic angle spinning nuclear magnetic resonance spectra of ²⁷Al nuclei in natrolite (Na₂Al₂Si₃O₁₀· 2H₂O) have been studied. The influence of water molecules and sodium ions mobility on the shape of the ²⁷Al NMR spectrum and framework dynamics have been discussed The temperature dependences of the spin–lattice relaxation times T₁ of ²⁷Al nuclei in natrolite have also been studied. It has been shown that the spin–lattice relaxation of the ²⁷Al is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H₂O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the ²⁷Al nuclei only at low temperatures (<270 K).     

EPR Spectroscopy of Impurity Thulium Ions in Yttrium Orthosilicate Single Crystals

The frequency-field and orientation dependences of the electron paramagnetic resonance (EPR) spectra are measured for impurity Tm³⁺ ions in yttrium orthosilicate (Y₂SiO₅) single crystals by stationary EPR spectroscopy in the frequency range of 50–100 GHz at 4.2 K. The position of the impurity ion in the crystal lattice and its magnetic characteristics are determined. The temperature dependences of the spin–lattice and phase relaxation times are measured by pulse EPR methods in the temperature range of 5–15 K and the high efficiency of the direct single-phonon mechanism of spin–lattice relaxation is established. This greatly shortens the spin–lattice relaxation time at low temperatures and makes impurity Tm³⁺ ions in Y₂SiO₅ a promising basis for the implementation of high-speed quantum memory based on rare-earth ions in dielectric crystals.     

X-band Electron Spin Relaxation Times for Four Aromatic Radicals in Fluid Solution and Comparison with Other Organic Radicals

X-band electron spin relaxation times of BDPA (1:1 α,γ-bisdiphenylene-β-phenylallyl), galvinoxyl 2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy, DPPH (2,2-diphenyl-1-picrylhydrazyl) and thianthrene radicals in fluid solution were measured by electron spin echo and inversion recovery at ambient temperature. Tumbling correlation times are estimated to be in the range of 20–30 ps. In this fast tumbling regime T ₁ ~ T ₂. Relaxation times are compared with previously reported values for symmetrically substituted triarylmethyl, semiquinone, and nitroxide radicals. The concentration dependence of spin lattice relaxation for neutral BDPA in toluene is about 10³ times greater than for anionic trityl radicals in water. T ₁ decreases in the order carbon-center BDPA > galvinoxyl > DPPH > thianthrene. The dominant relaxation mechanisms are proposed to be a local mode for BDPA, spin rotation, local mode and modulation of anisotropic proton hyperfine coupling for galvinoxyl, modulation of anisotropic nitrogen hyperfine for DPPH, and spin rotation plus modulation of anisotropic proton hyperfine coupling for thianthrene.     

Structural Phase Transition of Perovskite-Type N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>CdBr<Subscript>3</Subscript> Studied by MAS NMR and Static NMR

The structural geometry change in the perovskite-type N(CH₃)₄CdBr₃ single crystal near the phase transition temperature of T _C = 390 K was investigated using magic angle spinning nuclear magnetic resonance techniques. For ¹H and ¹³C nuclei, the temperature dependences of their chemical shift, spectral intensity, and spin–lattice relaxation time (T _1ρ) in the rotating frame were obtained and analyzed. While the chemical shift and T _1ρ of ¹H showed change near T _C, those of ¹³C did not. In addition, the ¹¹³Cd spin–lattice relaxation time T ₁ in the laboratory frame near T _C show no evidence of anomalous change near the phase transition temperature, which coincides with the measured changes in the ¹H T _1ρ. The driving force for this phase transition was connected to the ¹H in the CH₃ groups.     

27Al Magic Angle Spinning Nuclear Magnetic Resonance Study of Al1−x Cr x K(SO4)2·12H2O (x = 0, 0.07, and 0.2)

The physical properties of Al_1?x Cr _x K(SO₄)₂·12H₂O (x = 0, 0.07, and 0.2) were studied as a function of temperature using magic angle spinning nuclear magnetic resonance for ²⁷Al. On the basis of the physical properties of pure AlK(SO₄)₂·12H₂O, the effects of partially replacing Al³⁺ with Cr³⁺ ions were examined. Molecular motion changed with the concentration of Cr³⁺ ions. The relaxation process near 320 K was found to undergo molecular motion as described by the Bloembergen–Purcell–Pound theory. The activation energies, phase transition temperatures, and spin–lattice relaxation times in the rotating frame T _1ρ changed with the concentration of paramagnetic ions.     

The Influence of Surfactant Self-Association on 23Na Relaxation in Aqueous Solutions and Silica Aqueous Dispersions

Nuclear magnetic resonance ²³Na studies were carried out for sodium octanoate in aqueous solution and in silica aqueous dispersion. The concentration and temperature dependencies of spin–lattice and spin–spin relaxation times of sodium were investigated for ternary systems and binary solutions. The results indicate that the specific chemical association between counterion and ion rather than electrostatic interaction takes place in all systems. The existence of premicellar aggregates in octanoate solution was found.     

Multifrequency time-resolved EPR (9.5GHz and 95GHz) on covalently linked porphyrin-quinone model systems for photosynthetic electron transfer: effect of molecular dynamics on electron spin polarization

Covalently linked porphyrin–quinone model systems for photosynthetic electron transfer were examined by using time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (0.34T/9.5GHz, X-band) and high field and frequency (3.4T/95GHz, W-band). The paramagnetic transients studied were the light-induced spin-correlated radical pair states of the donor–acceptor complex in polar solvents below the melting point and in the soft glass phase of a liquid crystal. It is shown that the systems form strongly exchange-coupled radical pairs, whose TREPR lineshapes are determined mainly by fast electron recombination together with both spin–lattice relaxation and modulation of the exchange interaction. Below the melting point the spin–lattice relaxation rate naturally slows down, but that of the spin on the quinone site is still of the order of 10⁶ s^-1. Most probably this is due to contributions from spin–rotation interaction, and dependent on the molecular orientation with respect to the magnetic field. This relaxation anisotropy is related to anisotropic motion of the quinone site in the solvent cage. The results allow conclusions to be drawn concerning the molecular dynamics and flexibility of the systems. To yield long-lived radical pair states that would mimic photosynthetic electron transfer, the two mechanisms described, modulation of exchange and spin–rotation interactions, have to be suppressed by reducing the molecular flexibility of the complex.     

Growth,spectral, optical,and dielectric studies on novel semiorganic NLO single crystal: d-phenylglycine hydrochloride

Good quality novel semiorganic nonlinear optical single crystal of d-phenylglycine hydrochloride has been grown from the aqueous solution by low temperature solution growth method. X-ray diffraction reveals that the crystal crystallises into orthorhombic system with noncentrosymmetric space group P2₁2₁2₁. Experimental parameters are evaluated based on single-crystal XRD and the calculated values of the polarisability were compared with the values of polarisability using Clausius–Mossotti equation. The functional groups present in the grown crystal were confirmed by Fourier transform infrared spectral analysis. The ¹H and ¹³C FT–NMR has been recorded to elucidate the molecular structure. Ultraviolet–visible-near infra-red absorption studies on this crystal reveal that the minimum absorption region is around 228 nm. The optical band gap of the crystal was found to be 2.9 eV. The scanning electron microscope study has been carried out to determine the surface morphology of the grown crystal. Photoluminescence studies show that the material emits violet fluorescence. Thermal studies bring forth that the crystal is thermally stable up to 255 °C. Dielectric studies reveal that both the dielectric constant and dielectric loss decrease with the increase in frequency as like the typical semiorganic nonlinear optical crystals such as bisthiourea zinc chloride, bisthiourea cadmium chloride and l-arginine dihydrogen phosphate. Electrical conductivity measurements were carried out and the Arrhenius plot is used to determine the value of activation energy. The Kurtz powder analysis on the crystal confirms the existence of second harmonic generation properties. The SHG efficiency was found to be 1.15 times that of KDP crystal.     

The T 1 and T 2 Relaxation Times Distribution for the 35Cl and 14N NQR in Micro-composites and in Porous Materials

The paper describes the results of an experimental study of the ³⁵Cl and ¹⁴N nuclear quadrupole resonance in composite and porous materials, the influence of the environment and the crystallite size of powder on the nuclear quadrupole resonance line widths, as well as on the spin–spin and spin–lattice relaxation times. It is established that the spin–lattice relaxation time has a unimodal distribution, and the spin–spin relaxation time—bimodal distributions for all the investigated samples. The idealized distribution function of the relaxation times is obtained on the assumption that the concentration of inhomogeneities and relaxativity decreases with an increasing distance from the surface into the interior of the crystallite exponentially. It is shown that the model with the spin diffusion explains the shortening of the decay signal with decreasing grain size, but this is not confirmed by the experimental distribution of relaxation times obtained by means of the inverse Laplace transformation.     

EPR and Optical Absorption Study of Cu2+-Doped Diammonium Hexaaqua Magnesium Sulphate Single Crystals

Electron paramagnetic resonance (EPR) study of Cu²⁺ ions doped in diammonium hexaaqua magnesium sulphate single crystal over the temperature range of 4.2–320 K is reported. Copper enters the lattice substitutionally and is trapped at two magnetically equivalent sites. The spin Hamiltonian parameters are evaluated at 320, 300, 77, and 4.2 K. The angular variations of the resonance lines in three mutually perpendicular planes ab, bc* and c*a are used to determine principal g and A values. The observed spectra are fitted to a spin Hamiltonian of rhombic symmetry with parameters of Cu²⁺ at 77 and 4.2 K: g _xx  = 2.089, g _yy  = 2.112, g _zz  = 2.437 (±0.002) and A _xx  = 38, A _yy  = 14, A _zz  = 110 (±2) × 10^?4 cm^?1. The ground state wave function of Cu²⁺ ion in this lattice is determined. The g-factor anisotropy is calculated and compared with the experimental value. The optical absorption spectra of the crystal are also recorded at room temperature. With the help of assigned bands the crystal-field parameters (Dq, Ds and Dt) are evaluated. By correlating the optical and EPR data, the nature of bonding in the complex is discussed. The temperature dependence of the g values is explained to conclude the occurrence of both static and dynamic Jahn–Teller effects over the temperature range of investigation.     

Room Temperature High-Field Spin Dynamics of NV Defects in Sintered Diamonds

Sintered oriented nanodiamond arrays with the extremely high concentrations of the nitrogen-vacancy (NV) centers (up to 10³ ppm) were investigated by the W-band (94 GHz) electron spin echo electron paramagnetic resonance techniques. The NV centers were fabricated by the high-pressure high-temperature sintering of detonation nanodiamonds (DND) without the post or prior irradiation of the samples. The processes of polarization and recovery of the equilibrium population of the spin sublevels by optical and microwave pulses have been examined at room temperature in high magnetic fields corresponding to the fine-structure transitions for the NV defects at 94 GHz (3,250–3,450 mT). A long spin coherence time of 1.6 μs and spin–lattice relaxation time of 1.7 ms were measured. The results were compared with those obtained on the NV centers fabricated by the irradiation and subsequent annealing of the commercially available bulk diamonds. It was shown that the relaxation characteristics of the NV defects were similar in the both types of the samples despite the extremely high concentrations of NV defects and isolated nitrogen donors in the sintered DND.     

Study of the effect of precipitant on the high-temperature phase stability of SrO-ZrO2 prepared by n-butanol soft-template method

SrO doped zirconia (20%) was synthesized by n-butanol soft-template method using both NaOH and ammonia solution as precipitants. The high-temperature phase stability was investigated following further heat treatment at 1000°C for 2 h. XRD and Raman spectra were used to characterize the crystal form of zirconia. In addition, TEM was used to characterize the dispersibility of SrO doped zirconia. The results indicated that the concentration of OH^? introduced into the ZrO₂ lattice was the main factor controlling the crystal form of nanosized zirconia. The NaOH solution precipitant could improve the dispersibility of SrO doped t-ZrO₂, and could also prevent the phase transformation of zirconia from t-ZrO₂ to m-ZrO₂ effectively.     

<Superscript>53</Superscript>Cr NMR study of CuCrO<Subscript>2</Subscript> multiferroic

The magnetically ordered phase of the CuCrO₂ single crystal has been studied by the nuclear magnetic resonance (NMR) method on ⁵³Cr nuclei in the absence of an external magnetic field. The ⁵³Cr NMR spectrum is observed in the frequency range ν_res = 61–66 MHz. The shape of the spectrum depends on the delay tdel between pulses in the pulse sequence τ_π/2–t _del–τ_π–t _del–echo. The spin–spin and spin–lattice relaxation times have been measured. Components of the electric field gradient, hyperfine fields, and the magnetic moment on chromium atoms have been estimated.     

Wide-band EPR spectroscopy of YAlO3: Tm3+ single crystals

The YAlO₃: Tm³⁺ single crystal has been studied on a wide-band EPR spectrometer. The EPR spectra of Tm³⁺ ions in the frequency range of 90–160 GHz have been detected for the first time. It has been confirmed that thulium ions substitute the position of Y³⁺ in the crystal lattice. The detected spectra have been described with the use of a spin Hamiltonian with the effective spin S = 1/2. A comparative analysis of the orientation of the magnetic axes of the Tm³⁺ paramagnetic center with earlier data on other rare-earth ions has been performed.     

14N PNQR investigation of the phase transition in s-triazine

¹⁴N PNQR frequencies, linewidths and spin lattice relaxation times were measured in s-triazine between 77° and 280°K. A second order phase transition was observed at 198.5°K. The temperature dependence and anisotropy of the relaxation times indicate a reorientation of the molecule around its three-fold axis.     

Lattice distortion (Peierls Transition) caused by spin interaction in the chaotic impurity system of a semiconductor

The effect of an elastic spontaneous distortion of the crystal lattice of a doped semiconductor Ge:As near the insulator–metal (IM) phase transition has been discovered. The effect is manifested in the electron spin resonance (ESR) of neutral As atoms as a splitting of the single resonance absorption line. It observed at electron concentrations in the range 0.8 < n/n_C < 1 at low temperatures T < 100 K (n_C = 3.7 × 10¹⁷ cm^‐3 is the critical electron concentration for the IM phase transition). The splitting is the strongest along each of the six [110] directions, which indicates that the local lattice distortion occurs just in these directions. As a result, a sample is possibly divided into separate domains differing in the directions of compressive or tensile deformations. A study of concentration, temperature, and angular dependences of the effect has shown that the phenomenon discovered can be understood in terms of the Peierls spin transition model.     

ODMR Study of Zn1−x Mn x Se/Zn1−y Be y Se and (Cd1−x ,Mn)Te/Cd1−y Mg y Te Diluted Magnetic Semiconductor Quantum Wells

The effects of microwave pumping with a frequency of 60 GHz on the magneto-optical properties of diluted magnetic semiconductors (DMSs) are studied in (Zn,Mn)Se/(Zn,Be)Se and (Cd,Mn)Te/(Cd,Mg)Te quantum wells. Resonant heating of the Mn²⁺ ions in the electron spin resonance conditions leads to an increase in the Mn-spin temperature, which exceeds the bath temperature by up to 5.2 K, as detected by the shift of exciton emission line and decrease of its integral intensity. Nonresonant heating mediated by free carriers is also observed through variation of the polarization degree of emission. Direct measurements of spin–lattice relaxation times for both materials using time-resolved optically detected magnetic resonance (ODMR) technique have been performed. The mechanisms of ODMR in nanostructures of DMSs are discussed.